Active suspension system with enhanced suspension control characteristics at on/off transition of suspension control

ABSTRACT

An active suspension system includes a pressure holding mechanism with a pressure holding valve means which is maintained in conductive state for a given period in which the fluid pressure in a working chamber is gradually adjusted toward a predetermined neutral pressure level after turning OFF of a power switch. The pressure holding valve is, on the other hand, maintained in non-conductive state for a given period until the fluid pressure is adjusted to the neutral pressure after turning ON of the power switch. The system further includes a detector for detecting resumption of power supply after a short period of turning OFF of the power supply. A control system is responsive to the detector for detecting the resumption of power supply after power OFF state for a period shorter than the given period for performing short period resumption mode control operation by maintaining the pressure holding valve at the conductive state and setting the instantaneous fluid pressure in the working chamber as an initial pressure level.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an active suspension system for an automotive vehicle. More specifically, the invention relates to an active suspension system with a suspension control system which can provides enhanced control characteristics at ON/OFF transition period of a power supply for assuring riding comfort upon re-starting of power supply short period after turning OFF.

2. Description of the Background Art

Japanese Patent First (unexamined) Publication (Tokkai) Heisei 1-122717 discloses one of typical construction of an active suspension system. The shown system includes a hydraulic cylinder disposed between a vehicular body and a vehicular wheel. A fluid pressure in the working chamber of the hydraulic cylinder is externally controlled by a hydraulic pressure circuit and a pressure control valve disposed in the pressure circuit. The shown system further has a power holding system for maintaining power supply for a given period after turning of a main switch of power source circuit. During the given period, the fluid pressure in the working chamber is gradually adjusted toward a predetermined neutral pressure. The system is further provided with a pressure holding mechanism for maintaining the fluid pressure in the working chamber at the neutral pressure while the power is not supplied to the control system. Namely, in the shown system, pressure control can be maintained in the given period for adjusting the fluid pressure in the working chamber at the neutral pressure. The control system is thereafter shut down with maintaining the pressure holding mechanism at the pressure holding position so as to maintain the pressure in the working chamber at the neutral pressure. The pressure holding mechanism includes a pilot operated operational one-way check valve which is designed to be maintained at shut-off position while the line pressure in the supply line of the pressure circuit is maintained lower than the neutral pressure. However, even in the modern technology, it is quite difficult to completely shut-off the pressure circuit and thus precisely maintain the fluid pressure in the working chamber at the neutral pressure. In other words, the fluid pressure can drop below the neutral pressure while the power supply is absent. Therefore, at the ON-set of the power supply to initiating pressure supply through the pressure circuit, the line pressure in the supply line is increased abruptly. However, since the operational check valve in the drain line is maintained shut-off position until the line pressure reaches the neutral pressure. As a result, at the initial stage of suspension control after resumption of power supply for the active suspension system, uncomfortable change of fluid pressure in the working chamber may be caused.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide enhanced suspension control characteristics at ON- and OFF-set transition of power supply.

In order to accomplish aforementioned and other objects, an active suspension system, according to the present invention, includes a pressure holding mechanism with a pressure holding valve means which is maintained in conductive state for a given period in which the fluid pressure in a working chamber is gradually adjusted toward a predetermined neutral pressure level after turning OFF of a power switch. The pressure holding valve is, on the other hand, maintained in non-conductive state for a given period until the fluid pressure is adjusted to the neutral pressure after turning ON of the power switch. The system further includes a detector means for detecting resumption of power supply after a short period of turning OFF of the power supply. A control system is responsive to the detecting means detecting the resumption of power supply after power OFF state for a period shorter than the given period fir performing short period resumption mode control operation by maintaining the pressure holding valve at the conductive state and setting the instantaneous fluid pressure in the working chamber as an initial pressure level.

According to one aspect of the invention, an active suspension system comprises:

a suspension system disposed between a vehicular body and a road wheel for damping energy for causing relative displacement between the vehicular body and the road wheel, the suspension system including means for defining a variable pressure chamber;

a fluid circuit for circulating pressurized fluid across the variable pressure chamber, the fluid circuit including a pressurized fluid source and a pressure control valve means for adjusting fluid pressure within the variable pressure chamber across a predetermined neutral pressure;

a flow control valve network disposed between the pressurized fluid source and the pressure control valve means in the fluid circuit for establishing and blocking fluid communication between the pressurized fluid source and the pressure control valve means;

a sensor means for monitoring vehicular driving parameter affecting vehicular attitude for producing a sensor signal indicative thereof;

a power supply circuit for supplying an electric power to the fluid pressure source for driving the latter;

a control means responsive to the sensor signal for controlling the pressure control valve for adjusting pressure in the variable pressure chamber for regulating vehicular height and vehicular attitude, the control unit being powered by the electric power supplied by the power supply circuit, the control unit being designed to maintain operation for a predetermined first period after shutting down of power supply from the power supply circuit for adjusting the fluid pressure in the vicinity of the neutral pressure and operating the flow control valve network at the blocking position for maintaining the pressure in the variable pressure chamber at the pressure in the vicinity of the neutral pressure, the control means being responsive to initiation of power supply for maintaining the flow control valve network at flow blocking position for a predetermined second period of time, and the control means being responsive to resumption of power supply within the first period for maintaining the flow control valve network at fluid communication establishing state and setting instantaneous fluid pressure as an initial pressure for starting suspension control.

According to another aspect of the invention, an active suspension system comprises:

a suspension system disposed between a vehicular body and a road wheel for damping energy for causing relative displacement between the vehicular body and the road wheel, said suspension system including means for defining a variable pressure chamber;

a fluid circuit, including a supply line for supplying pressurized fluid and a drain line for driving pressurized fluid, for circulating pressurized fluid through said supply line and said drain line across said variable pressure chamber, said fluid circuit including a pressurized fluid source and a pressure control valve means for adjusting fluid pressure within said variable pressure chamber across a predetermined neutral pressure;

a flow control means disposed between said pressurized fluid source and said pressure control valve means in said fluid circuit, said flow control means being operable between a first flow control mode for supplying line pressure for said pressure control valve at a first maximum flow rate and a second flow control mode for supplying line pressure for said pressure control valve at a second limited flow rate which is smaller than said first maximum flow rate.

a sensor means for monitoring vehicular driving parameter affecting vehicular attitude for producing a sensor signal indicative thereof;

a power supply circuit for supplying an electric power to said fluid pressure source for driving the latter so as to enable supply of pressurized fluid to said fluid circuit;

a control means responsive to said sensor signal for controlling said pressure control valve, said control means being operable in a first normal mode for adjusting pressure in said variable pressure chamber for regulating vehicular height and vehicular attitude, a second turn OFF transition mode initiated in response to shutting down of said power supply circuit for operating said flow restriction means at said first flow control mode for adjusting fluid pressure in said working chamber toward a first set pressure set in the vicinity of said neutral pressure during a predetermined first turn OFF transition period, and a third turn ON transition mode for operating said flow restriction means at said second flow control mode for adjusting fluid pressure in said working chamber toward a second set pressure which is set higher pressure than said neutral pressure in a given magnitude during a second ON transition period, and said control means being responsive to resumption of power supply within said first turning OFF transition period for maintaining said flow control valve means at said first flow control mode and setting instantaneous fluid pressure as an initial pressure for starting suspension control.

In either case, the control means may normally derive suspension control signal on the basis of said sensor signal so as to regulate vehicular height and vehicular attitude, said control means setting said control signal for a predetermined fixed value during said second turning ON transition period. In such case, the sensor means may comprise a height sensor for monitoring vehicular height level to produce a height level indicative sensor signal, and said control means derives said suspension control signal on the basis of a difference between said height level indicative signal value and a target height representative value, said control means set said target height representative value at a first normal value at a normal operation and a second smaller value smaller than said first normal value during said second turning ON transition period.

The control unit may further set an interval of height regulation control cycle at a first normal period and a second reduced period during said second turning ON transition period, which second reduced period is set to be shorter than said first normal period. Also, the control unit may further set a height adjustment magnitude for each height adjustment cycle, which height adjustment magnitude is set at a first normal value in the normal state of height control and a second increased value during said second turning ON transition period.

Preferably, the control unit adjusts said fluid pressure in said working chamber within a predetermined range set about a set pressure, during said first turning OFF transition period. Furthermore, the active suspension system may further comprise a fail-safe valve in said fluid circuit, and a fail detecting means for monitoring operation to detect failure of the active suspension system for producing a fail indicative signal, said control unit normally operates said fail safe valve to a first open position and responsive to said fail indicative signal for blocking fluid communication therethrough for forming closed circuit across said pressure control valve and said working chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment but are for explanation and understanding only.

In the drawings:

FIG. 1 is a diagrammatic illustration of the preferred embodiment of an active suspension control system, according to the present invention;

FIG. 2 is a block diagram showing a preferred construction of a control system associated with the preferred embodiment of the active suspension system of FIG. 1;

FIG. 3 is a chart showing relationship between current level of a suspension control command and fluid pressure in a working chamber in the preferred embodiment of the active suspension system;

FIG. 4 is a flowchart showing a rolling control routine for deriving the suspension control command.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, particularly to FIG. 1, the preferred embodiment of an active suspension system, according to the present invention, is designed to generally perform suspension control for regulating vehicular height level and vehicular attitude by suppressing relative displacement between a vehicular body and suspension members provided in front-left, front-right, rear-left and rear-right suspension mechanism 14FL, 14FR, 14RL and 14RR and rotatably supporting front-left, front-right. rear-left and rear-right wheels. The suspension mechanism as generally referred to will be hereafter represented by the reference numeral "14" Respective front-left, front-right, rear-left and rear-right suspension mechanisms 14FL, 14FR, 14RL and 14RR have hydraulic cylinders 26FL, 26FR, 26RL and 26RR which will be represented by the reference numeral "26" as generally referred to.

Each of the hydraulic cylinder 26 is disposed between the vehicular body and the suspension member to produce a damping force for suppressing relative displacement between the vehicular body and the suspension member. The hydraulic cylinder 26 generally comprises an essentially enclosed cylindrical cylinder body 26a defining therein an enclosed chamber. A thrusting piston 26c is thrustingly and slidably disposed within the enclosed chamber of the hydraulic cylinder 26 for defining in the latter a working chamber 26d. The piston 26c is connected to the associated one of suspension member 24 via a piston rod 26b. A suspension coil spring may also be provided in respective of suspension mechanisms. However, in contrast to the normal or ordinary suspension systems, the suspension coil spring to be employed in the shown type of the suspension system is not required a resilient force in a magnitude required in the ordinary suspension system and only required the resilient force necessary for maintaining the vehicular body about the suspension member.

The working chamber 26d of the hydraulic cylinder 26 is connected one of pressure control valves 28FL, 28FR, 28RL and 28RR via a pressure control line 38. The pressure control valve 28FL, 28FR, 28RL and 28RR will be hereafter represented by the reference numeral "28" as generally referred to. The pressure control valve 28 has a control port 28c communicated with the working chamber 26d via the pressure control line 38. The pressure control valve 28 also has an inlet port 28s and a drain port 28r. The inlet port 28s of the pressure control valve 28 is connected to o a pressure source unit 16 via a supply line 35, and the drain port 28r thereof is communicated with a drain line 37. The pressure control valve 28 incorporates a proportioning solenoid 26e for adjusting valve position according to magnitude of suspension control signal I_(FL), I_(FR), I_(RL) and I_(RR) supplied from the control unit 100. The suspension control signal I_(FL), I_(FR), I_(RL) and I_(RR) are current signal having a variable current value representative of commanding pressure in the working chamber. A branch circuit is provided for connecting the working chamber 38 to a pressure accumulator 34 via a flow path 33 and a flow restricting means, such as an orifice 32. This pressure accumulator 34 may be hereafter referred to as "low pressure accumulator". Another pressure accumulators 20F and 20R are provided in the supply line 35 for accumulating the excessive pressure generated in the pressure source unit 16.

The pressure control valves 2 comprise, though it is not clearly shown in FIG. 1, electrically or electromagnetically operable actuators, such as a proportioning solenoids. The hydraulic cylinder 26 and the pressure control valve 28 may be of any suitable constructions for adjusting damping characteristics with sufficiently high response. Typical constructions of the hydraulic cylinder 26 and the pressure control valve 28 have been disclosed in the following prior applications or publications:

U.S. patent application Ser. No. 052,934, filed on May 22, 1989;

U.S. patent application Ser. No. 059,888, filed on June 9, 1987, corresponding European Patent Application has been published as First Publication No. 02 49 209;

U.S. patent application Ser. No. 060,856, filed on June 12, 1987, corresponding European Patent Application has been published as First Publication No. 02 49 227;

U.S. patent application Ser. No. 060,909, filed on June 12, 1987;

U.S. patent application Ser. No. 060,911, filed on June 12, 1987;

U.S. patent application Ser. No. 176,246, filed on Mar. 31, 1988, the corresponding European Patent Application has been published as First Publication No. 02 85 153;

U.S. patent application Ser. No. 178,066, filed on Apr. 5, 1988, the corresponding European Patent Application has been published as First Publication No. 02 86 072;

U.S. patent application Ser. No. 167,835, filed on Mar. 4, 1988;

U.S. patent application Ser. No. 244,008, filed on Sept. 14, 1988;

U.S. patent application Ser. No. 255,560, filed on Oct. 11, 1988;

U.S. patent application Ser. No. 266,763, filed on Nov. 3, 1988;

U.S. patent application Ser. No. 261,870, filed on Oct. 25, 1988;

U.S. patent application Ser. No. 263,764, filed on Oct. 28, 1988;

U.S. patent application Ser. No. 277,376, filed on Nov. 29, 1988;

U.S. patent application Ser. No. 303,338, filed on Jan. 26, 1989;

U.S. patent application Ser. No. 310,130, filed on Mar. 22, 1989;

U.S. patent application Ser. No. 327,460, filed on Mar. 22, 1989;

U.S. patent application Ser. No. 303,339, filed on Jan. 26, 1989;

U.S. patent application Ser. No. 331,602, filed on Mar. 31, 1989;

U.S. patent application Ser. No. 331,653, filed Mar. 31, 1989;

U.S. patent application Ser. No. 364,477, filed on June 12, 1989;

U.S. patent application Ser. No. 365,468, filed on June 12, 1989.

The disclosures of the above-identified copending U.S. Patent Applications and corresponding publications are herein incorporated by reference for the sake of disclosure.

The pressure source unit 16 includes a fluid pump 161 which is drivingly associated with the output shaft 161a of an automotive engine 161b to be driven by the driving torque of the engine. The fluid pump 161 is connected to a fluid reservoir 162 via a suction line 163 to suck a working fluid stored in the fluid reservoir 162. The pressure source unit 16 also includes a surge valve 164 for preventing surge flow of the pressurized fluid, a pressure regulating accumulator 165, a fluid filter 166 and a by-pass path with an one-way check valve 167.

The shown circuit includes a pressure holding section 17 which includes a fluid flow rate adjusting stage 170. The flow rate adjusting stage 170 includes a flow control valve 171. The pressure holding section 17 also includes a pilot operated operational check valve 172, a pressure relief valve 173. The pressure relief valve 173 is set a predetermined set pressure for determining a maximum level of line pressure in the supply line 35. The operational check valve 172 is connected to the supply line in a pilot port for introducing therefrom the line pressure as a pilot pressure P_(P). In general, the operational check valve 172 is responsive to the pilot pressure P_(P) higher than the aforementioned neutral pressure P_(N) to permit the working fluid in the drain line 37 to flow returning to the fluid reservoir 162 via a fluid filter 168. On the other hand, when the pilot pressure P_(P) introduced from the supply line 35 is lower than the neutral pressure P_(N), the operational check valve 172 is maintained in flow blocking position for blocking fluid flow through the drain line 37.

The flow control valve 171 incorporates an electromagnetic actuator 171a connected to the control unit 100 and is designed to be controlled the valve position by a flow control signal CS₂ supplied from the control unit. In brief, the flow control valve 171 is designed to be placed at flow blocking position for a given period after turning of an ignition switch (not shown) from OFF to ON. While the flow control valve 171 is maintained in flow blocking position, the pressurized fluid is supplied only through a flow restriction orifice 171b. Therefore, at this valve position, limited flow rate of the working fluid flows through the supply line 35.

A fail-safe valve 18 is provided downstream of the flow rate adjusting section 17. The fail-safe valve 18 comprises a spring off-set type four port, two position valve which has a P port connected to an one-way check valve 174 in the flow rate adjusting section 17. The fail-safe valve 18 also has A port connected to the inlet ports 28s of respective pressure control valves 28, B port connected to the drain ports 28r of the pressure control valves 28, and B port connected to inlet port of the operational check valve 172. The fail-safe valve 18 incorporates an electromagnetic actuator 181 connected to the control unit to be controlled the valve position by a fail-safe signal CS₁.

While the fail-safe signal CS₁ is maintained HIGH level to maintain the actuator 181 at energized state the fail-safe valve 18 establishes fluid communication between the P port and A port for supplying the line pressure to the pressure control valves 28. At the same time, the B port and R port are also communicated to permit fluid in the drain line 37 to flow therethrough. On the other hand, when the fail-safe signal CS₁ is maintained LOW level to maintain the actuator 181 deenergized position, the fail-safe valve 18 is placed at the flow blocking position by a force of a return spring 182. At this position, the fluid flow through the supply line 35 is thus blocked. Also, since the R port is disconnected from the B port, the fluid in the drain line 37 is permitted only through a flow restriction orifice 183.

At the position between the pressure control valve 28 and the fail-safe valve 18, the supply line 35 is branched into front-left, front-right, rear-left and rear-right branches 35FL, 35FR, 35RL and 35RR respectively connected to the inlet ports 28s of the front-left, front-right, rear-left and rear-right pressure control valves 28FL, 28FR, 28RL and 28RR. Similarly, the drain line 37 is branched into front-left, front-right, rear-left and rear-right branches 37FL, 37FR, 37RL and 37RR respectively connected to the drain ports 28r of the front-left, front-right, rear-left and rear-right pressure control valves 28FL, 28FR, 28RL and 28RR. The pressure accumulator 20F is connected to the upstream of a junction of the front-left and front-right branches 35FL and 35FR. Also, the pressure accumulator 20R is connected to the upstream of a junction of the rear-left and rear-right pressure control valves 35RL and 35RR. On the other hand, at a downstream of the junction between the front-left and front-right branches 37FL and 37FR of the drain line 37 is connected to the pressure accumulator 18F via an one-way check valve 36F. Similarly, at a downstream of the junction between the rear-left and rear-right branches 37RL and 37RR of the drain line 37 is connected to the pressure accumulator 18R via an one-way check valve 36R. Between the junctions where the branched drain lines 37F and 37R are connected to the pressure accumulators 18F and 18R and the B port of the fail-safe valve 18, flow restriction orifices 22F and 22R and pressure accumulators 24F and 24R are provided for absorbing back pressure in the drain line.

The actuators of the pressure control valves 28, the flow control valve 171 and the fail-safe valve 18 are connected to a microprocessor based control unit 100. The control unit 100 is connected to a lateral acceleration sensor 102, a longitudinal acceleration sensor 104, height sensors 106FL, 106FR, 106RL and 106RR. As is well known, the lateral acceleration sensor 102 monitors a lateral acceleration exerted on the vehicular body to produce a lateral acceleration indicative signal g_(y). Similarly, the longitudinal acceleration sensor 104 monitors a longitudinal acceleration exerted on the vehicle body to produce a longitudinal acceleration indicative signal g_(x). The height sensors 106FL, 106FR, 106RL and 106RR are provided at respective positions where front-left, front-right, rear-left and rear-right suspension mechanisms 14FL, 14FR, 14RL and 14RR are provided. The height sensors 106FL, 106FR, 106RL and 106RR monitor relative distance between the vehicular body and associated one of the suspension members of the suspension mechanisms 14FL, 14FR, 14RL and 14RR to produce front-left, front-right, rear-left and rear-right height indicative signals H_(FL), H_(FR), H_(RL) and H_(RR).

As shown in FIG. 2, the control unit 100 comprises a microcomputer 110 which includes an input/output interface 112, a microprocessor 114 and a memory unit 116. The lateral acceleration sensor 102 is connected to the input/output interface 112 of the microcomputer 110 via an analog-to-digital (A/D) interface 120. The A/D converter 120 converts the analog form lateral acceleration indicative signal g_(y) into a lateral acceleration indicative digital signal.

On the other hand, the longitudinal acceleration sensor 104 is connected to the input/output interface 112 via an A/D converter 122. The A/D converter 122 converts the analog form longitudinal acceleration indicative signal g_(x) into a digital form longitudinal acceleration indicative digital signal. Also, respective of the front-left, front-right, rear-left and rear-right height sensors 106FL, 106FR, 106RL and 106RR are connected to the input/output interface 112 via A/D converters 124FL, 124FR, 124RL and 124RR. The A/D converters 124FL, 124FR, 124RL and 124RR converts analog form front-left, front-right, rear-left and rear-right height indicative signals H_(FL), H_(FR), H_(RL) and H_(RR) into digital form height indicative digital signals. The microcomputer 110 processes the lateral acceleration indicative digital signals, the longitudinal acceleration digital signals and the front-left, front-right, rear-left and rear-right height indicative digital signals for deriving front-left, front-right, rear-left and rear-right suspension control commands P_(FL), P_(FR), P_(RL) and P_(RR). The suspension control commands P_(FL), P_(FR), P_(RL) and P_(RR) are converted into voltage signals V_(FL), V_(FR), V_(RL) and V_(RR) respectively representative of the commanded value of the suspension control commands, by digital-to-analog (D/A) converters 126FL, 126FR, 126RL and 126RR. The voltage signals V_(FL), V_(FR), V_(RL) and V_(RR) are supplied to driver circuits 128FL, 128FR, 128RL and 128RR. The driver circuits 128FL, 128FR, 128RL and 128RR derives the suspension control signals I_(FL), I_(FR), I_(RL) and I_(RR) which are current signal having current level corresponding to the magnitude of adjustment of the fluid pressure.

As can be seen from FIG. 3, the current value of the suspension control signal as the current signal is variable between a predetermined minimum value I_(MIN) and a predetermined maximum value I_(MAX). The fluid pressure becomes minimum P_(MIN) at the minimum value I_(MIN) of the suspension control command and becomes maximum P_(MAX) at the maximum value I_(MAX) of the suspension control command. The minimum value I_(MIN) of the suspension control command is set in view of noise possible superimposed on the suspension control current. As can be seen, the fluid pressure in the working chamber 26d of the hydraulic cylinder 26 linearly varies between the maximum pressure level P_(MAX) and the minimum pressure level P_(MIN) across a predetermined neutral pressure P_(N) corresponding to the current level I_(N).

A fail detector 130 is also provided in the suspension control system for monitoring operating condition and detecting abnormal condition of the system to input an abnormality detection signal AS to the input/output interface 112. Furthermore, a power circuit 132 is connected to the control unit 100. The power circuit 132 includes a power regulator 134 and an ignition relay 136. The ignition relay 136 has a relay coil 136a [E] connected to a vehicular battery 138. The other end of the relay coil 136a is connected to collector electrodes of a pair of transistors Q₁ and Q₂. The transistor Q₁ has the base electrode connected to the battery 138 via an ignition switch 140. On the other hand, the transistor Q₂ has the base electrode connected to the input/output interface 112 to receive therefrom a self-holding signal S_(S). The microprocessor 110 is also connected to a watch dog timer 142 which receives periodic signal internally produced in the microprocessor for checking the operation of the control unit. The watch dog timer 142 is connected to NMI (non-maskable interrupt) terminal of the microprocessor for supplying an abnormality indicative signal when failure of the control unit is detected.

In brief of the operation, the control unit 100 initiate operate in response to turning on of the ignition switch 140. At the initial stage, whole system is initialized. At this initialization stage, the fail-safe signal CS₁ and the flow control signal CS₂ are turned into HIGH level. At the same time, the suspension control command values P_(FL), P_(FR), P_(RL) and P_(RR) are set at a predetermined initial value P_(K). The suspension control commands P_(FL), P_(FR), P_(RL) and P_(RR) having the predetermined value P_(K) is thus supplied to the driver circuits 128FL, 128FR, 128RL and 128RR via respectively corresponding D/A converters 126FL, 126FR, 126RL and 126RR. For a predetermined ON-set period, the values of the suspension control commands P_(FL), P_(FR), P_(RL) and P_(RR) are maintained at the predetermined initial value P_(K). After expiration of the On-set period, the height indicative data H_(FL), H_(FR), H_(RL) and H_(RR) are read out. Then height regulating control operation is performed by the the control unit 100. On the height regulating control operation, the respective of the height indicative data H_(FL), H_(FR), H_(RL) and H_(RR) is compared with a predetermined target height H_(S) so that the suspension control command P_(FL), P_(FR), P_(RL) and P_(RR) necessary for adjusting the height to the target height are generated. Also, anti-rolling and anti-pitching control for regulating vehicular attitude is performed on the basis of the lateral and longitudinal acceleration data Y_(G) and X_(G). In the anti-rolling and anti-pitching control, anti-rolling control command P_(YG) and anti-pitching control command P_(XG) are derived for suppressing vehicular rolling and pitching motion.

On the other hand, when the ignition switch 140 is turned OFF, the suspension control commands P_(FL), P_(FR), P_(RL) and P_(RR) are gradually adjusted toward the predetermined initial value P_(K). Simultaneously, the position of the ignition switch 140 is continuously monitored in order to detect turning ON the same again. When the ignition switch is turned ON before the suspension control commands P_(FL), P_(FR), P_(RL) and P_(RR) are adjusted to the predetermined value P_(K), then the current suspension command values upon ON-set of the ignition switch are taken as the initial command value.

On the other hand, the microprocessor 110 is responsive to the abnormality indicative signal AS from the fail detector or from the watch dog timer to initiate fail-safe operation. In the fail-safe operation, the fail-safe signal CS₁ is turned into LOW level to place the fail-safe valve 18 at the flow blocking position.

The following is a detail of the suspension control routine to be executed by the control unit 100 in the shown embodiment of the active suspension system, according to the invention.

FIG. 4 shows a routine to be executed by the microprocessor 110 in the control unit 100 of the shown embodiment of the active suspension system. As can be appreciated, the shown routine may be programmed as one of interrupt routines to be executed by interrupting a main program which governs a plurality of routines. Therefore, the shown routine may be executed periodically at a given interval.

Initially, the shown routine is triggered in response to turning ON of the ignition switch 140. Namely, upon ON-set of the ignition switch 140, the transistor Q₁ of the power circuit 132 is turned ON. In response to this, the ignition relay 136 is turned ON. Therefore, power supply for the control unit 100 is initiated.

Immediately after starting execution, a short period re-starting indicative flag F_(S) which represents turning ON of the ignition switch after a shorter period that a predetermined period after turning OFF of the ignition switch 140 to establish pressure holding position of the pressure holding mechanism 17, is checked at a step 1002. When the short period re-starting indicative flag F_(S) is not set as checked at the step 1002. Then, judgement can be made that the pressure holding mechanism 17 is in the pressure holding state. Then, the fail-safe signal CS₁ is switched into HIGH level to place the fail-safe valve at open position for permitting fluid flow through the supply line 35 and the drain line 37, at a step 1004. Also, at a step 1006, the flow control signal CS₂ is switched into HIGH level to place the flow control valve 170 at the flow blocking position. Therefore, the pressurized fluid flows only through the flow restriction orifice 171b for supplying limited flow rate of the working fluid for the pressure control valves 28FL, 28FR, 28RL and 28RR. Thereafter, a initial period timer N_(I) is incremented at a step 1008. Subsequently, the suspension control commands P_(FL), P_(FR), P_(RL) and P_(RR) are set at an initial values P_(FL) ', P_(FR) ', P_(RL) ' and P_(RR) ' at a step 1010.

Here, in general, the suspension control command values P_(FL), P_(FR), P_(RL) and P_(RR) are derived on the basis of the neutral pressure P_(N), height control value PH_(FL), PH_(FR), PH_(RL) and PH_(RR) which are derived on the basis of the difference between the target height H_(S) and the height indicative data H_(FL), H_(FR), H_(RL) and H_(RR), the anti-rolling control value P_(YG), the anti-pitching control value P_(XG), and off-set command value PO_(FL), PO_(FR), PO_(RL) and PO_(RR) which represents required off-set pressure from the neutral pressure P_(N) set for compensating lag time of pressure adjustment. The manner of derivation of the suspension control commands P_(FL), P_(FR), P_(RL) and P_(RR) can be expressed by the following equations: ##EQU1## Here, at the initial stage upon ON-set of the ignition switch 140, the height control values PH_(FL), PH_(FR), PH_(RL) and PH_(RR), the anti-rolling control value P_(YG) and the anti-pitching control value P_(XG) are maintained zero (0). Therefore, assuming the initial off-set value PO_(FL), PO_(FR), PO_(RL) and PO_(RR) are respectively P_(I), the foregoing equation can be modified for deriving the initial suspension control command values P_(FL) ', P_(FR) ', P_(RL) ' and P_(RR) ' ##EQU2## Therefore, at a step 1010, the suspension control command values P_(FL) ', P_(FR) ', P_(RL) ' and P_(RR) ' are derived by the foregoing equations (5) to (8). Then, the suspension control commands P_(FL), P_(FR), P_(RL) and P_(RR) with the derived values P_(FL) ', P_(FR) ', P_(RL) ' and P_(RR) ' are output at a step 1012. Thereafter, at a step 1014, the initial period timer value N_(I) is compared with a predetermined set initial period value N_(SI) which represents an ON-set period for maintaining the pressure holding mechanism 17 at the pressure holding position after switching of the ignition switch 140 into ON position. In practice, the ON-set period is set at several seconds in the preferred embodiment. When the initial period timer value N_(I) is smaller than the set initial period value N_(SI), the process returns to the step 1008. Therefore, the process through the steps 1008 to 1014 is repeated until the initial period timer value N_(I) reaches the set initial period value N_(SI). When the initial period timer value N_(I) becomes greater than or equal to the set initial period value N_(SI) as checked at the step 1014, the flow control signal CS₂ is switched into LOW level for operating the flow control valve 171 in the pressure holding mechanism 17 into open position. Thereafter, the initial stage indicative flag F₁ is set at a step 1018.

On the other hand, when the short period re-starting indicative flag F_(S) as checked at the step 1002 is set, then the off-set command values PO_(FL), PO_(FR), PO_(RL) and PO_(RR) which are generally represented by PO_(j), are derived at a step 1020. In practice, the off-set command values PO_(FL), PO_(FR), PO_(RL) and PO_(RR) are calculated from the following equations:

    PO.sub.j =P.sub.j -P.sub.N

where P_(j) generally represents P_(FL), P_(FR), P_(RL) and P_(RR)

After the process at the step 1020, process jumps to the step 1018.

AT a step 1022, the off-set command values PO_(j) is checked if the value thereof is zero. When, the odd-set command value PO_(j) is zero as checked at the step 1022, then, the height indicative data H_(FL), H_(FR), H_(RL) and H_(RR) which will be generally represented by H_(j), at a step 1024. On the other hand, when the off-set command value PO_(j) is not zero as checked at the step 1022, the off-set command value is reduced by subtracting ΔP₁ at a step 1026. After process at the step 1026, process goes to the step 1024.

At a step 1028, a height difference H_(E) is derived by subtracting the target height value H_(S) from the height indicative data H_(j) at a step 1028. Then, the initial stage indicative flag F₁ is checked at the step 1030. When the initial stage indicative flag F₁ is set at the step 1030, a height adjustment threshold value .sub.[E]T[-]TH and the target height value H_(S) are set smaller value than T₁ and H₁. On the other hand, the a height variation indicative value ΔH is set at greater values than ΔH₁, at a step 1032. On the other hand, when the initial stage indicative flag F₁ is not set as checked at the step 1030, then, height adjustment judgment period T_(TH), the target height value H_(S) and a height variation indicative value ΔH are respectively set at greater values T₀ (>T₁), H₁ (>H₀) and ΔH₁ (>ΔH₁) at a step 1034.

After process of one of the steps 1032 and 1034, the absolute value of the height difference |H_(E) 1| is compared the target height value H_(S), at a step 1036. When the absolute value |H_(E) | of the height difference is greater than the target height H_(S), a height adjustment timer T_(H) is incremented by one at a step 1038. On the other hand, when the absolute value |H_(E) | of the height difference is smaller than or equal to the target height value H_(S), the height adjustment timer value T_(H) is cleared at a step 1040. Thereafter, the initial stage indicative flag F₁ is rest at a step 1042.

Then, at a step 1044, the height adjustment timer value T_(H) is compared with the height adjustment threshold value T_(TH) at a step 1044. If the height adjustment timer value T_(H) is greater than or equal to the height adjustment threshold value T_(TH), then, the height difference H_(E) is checked whether it is zero (0) or not at a step 1046. Thereafter, at a step 1048, the height control command value PH_(FL), PH_(FR), PH_(RL) and PH_(RR) which will be generally represented by PH_(j), are derived by subtracting the height variation indicative value ΔH from the previously derived height control command value derived in the immediately preceding execution cycle. Otherwise, at a step 1050, the height control command value PH_(j), are derived by adding the height variation indicative value ΔH to he previously derived height control command value derived in the immediately preceding execution cycle.

When the height adjustment timer value T_(H) is smaller than the height adjustment threshold value T_(TH) as checked at the step 1044 or after the process at one of the steps 1048 and 1050, the lateral acceleration indicative data Y_(G) which is derived from the lateral acceleration indicative signal y_(g) by A/D conversion, is read out at a step 1052. Based on the lateral acceleration indicative data Y_(G), a rolling magnitude indicative data Y_(G) is derived. Practically, the rolling magnitude indicative data Y_(G) is derived by subtracting a neutral position lateral acceleration value Y_(G0) at the position where the lateral acceleration on the vehicular body is zero (0) from the lateral acceleration indicative data Y_(G), at a step 1054. By this, the rolling magnitude indicative data Y_(G) represents both of the vehicular rolling magnitude and the vehicular rolling direction. Thereafter, by multiplying the rolling magnitude indicative data Y_(G) with a control coefficient K_(Y), anti-rolling control command value P_(YG) is derived at a step 1056. The longitudinal acceleration indicative data X_(G) which is derived from the longitudinal acceleration indicative signal x_(g) by A/D conversion, is read out at a step 1058. Based on the longitudinal acceleration indicative data X_(G), a rolling magnitude indicative data X_(GR) is derived. Practically, the rolling magnitude indicative data X_(GR) is derived by subtracting a neutral position longitudinal acceleration value X_(G0) at the position where the longitudinal acceleration on the vehicular body is zero (0) from the longitudinal acceleration indicative data X_(G), at a step 1060. By this, the rolling magnitude indicative data X.sub. GR represents both of the vehicular rolling magnitude and the vehicular rolling direction. Thereafter, by multiplying the rolling magnitude indicative data X_(GR) with a control coefficient K_(X), anti-rolling control command value P_(XG) is derived at a step 1062.

After the process at the step 1062, the suspension control command values P_(FL), P_(FR), P_(RL) and P_(RR) are derived at a step 1064. The suspension control command P_(FL), P_(FR), P_(RL) and P_(RR) with the control valve thus derived are output at a step 1066.

At a step 1068, the abnormality indicative signals from the fail detector 130 and the watch dog timer 142 are checked. If abnormality is detected at the step 1068, the fail-safe signal CS₁ is set at LOW level at the step 1070. At the same time, a self-holding signal S_(S) for maintaining power supply for a predetermined OFF-set period after turning OFF of the ignition switch 140, is switched into LOW level at the step 1070. Then, process goes END.

On the other hand, when the abnormality cannot be detected as checked at the step 1068, the position of the ignition switch 140 is checked at a step 1072. If OFF state of the ignition switch 140 is detected at the step 1072, then process returns to the step 1022. On the other hand, if the ignition switch 140 is in ON position, the short period re-starting indicative flag F_(S) is set at a step 1074.

At a step 1076, an end timer value T_(E) is incremented. Then, the end timer value T_(E) is compared with a first end timer threshold T_(ES1) at a step 1078. When, the end timer value T_(E) is smaller than the first end timer threshold T_(ES1), the suspension control command value P_(j) is checked whether it is within a predetermined range ±α of a predetermined set pressure P_(K) at a step 1080. When the suspension control command P_(j) is not within the predetermined range (P_(K) -α≦P_(j) ≦P_(K) +α), then check is performed whether the suspension control command value P_(j) is greater than the set pressure P_(K) at a step 1082. When the suspension control command P_(j) is greater than the set pressure P_(K), then, the off-set command value PO_(j) is set at a predetermined value -P₂, at a step 1084. On the other hand, when the suspension control command value P_(j) is greater than the set pressure P.sub. K, then the off-set command value PO_(j) is set at a predetermined value +P₂ at a step 1086. After the process at one of the steps 1084 and 1086 or when the suspension control command value P_(j) is within the predetermined range (P_(K) -α≦P_(j) ≦P_(K) +α) as checked at the step 1080, then check is performed whether the ignition switch 140 is in OFF position at a step 1088. If the ignition switch position as checked at the step 1088 is OFF, then process returns to the step 1076. On the other hand, when the ignition switch at ON position is detected at the step 1088, then process returns to the step 1002. On the other hand, when the end timer value T_(E) is greater than or equal to the first end timer threshold T_(ES1) as checked at the step 1078, the short period re-strarting indicative flag F_(S) is reset at a step 1090. Thereafter, the end timer value T_(E) is compared with a second end timer threshold T_(ES2) which represents longer period than the first end timer threshold T_(ES1) and defines a period for operating the pressure holding mechanism 17 at the pressure holding position. When the end timer value T_(E) is smaller than the second end timer threshold T_(ES2) as checked at the step 1092, process goes back to the step 1076. On the other hand, when the end timer value T_(E) is greater than the second end timer threshold T_(ES2) as checked at the step 1092, the end timer value T_(E) is cleared at a step 1094 and the self-holding signal S_(S) is switched into LOW level at a step 1096. Thereafter, process goes END.

When the vehicle stops at a flat road with maintaining the ignition switch 140 at OFF position for period longer than the second end timer threshold T_(ES2), the pressure source unit 16 is maintained inoperative position because of stopping of the engine. Therefore, the line pressure in the supply line 35 supplied from the pressure source unit 16 is maintained zero. At this condition, due to the line pressure in the supply line lower than or equal to the neutral pressure, the operational one-way check valve 172 is maintained at shut-off state. At this time, the short period re-starting indicative flag F_(S) in the memory unit 116 is maintained in reset state.

By turning ON of the ignition switch 140 at this position, power supply is initiated to resume control operation of the control unit 100. At the same time, the engine starts to run for driving the fluid pump 161. Therefore, supply pressure from the pressure source unit 16 is gradually increased according to increasing of the engine revolution speed. Also, the microprocessor 110 of the control unit 100 initiate suspension control operation.

At this state, since the short period re-starting indicative flag F_(S) is maintained at reset state. Therefore, both of the flow control signal CS₂ and the fail-safe signal CS₁ are set at HIGH level during initialization process at the steps 1004 and 1006. Therefore, the fail-safe valve 18 is operated into open position and the flow control valve 171 is operated into shut-off position. As a result, the fluid pressure supplied from the pressure source unit 16 is supplied to the pressure control valve 28 via the flow restriction orifice 171b. Therefore, limited flow rate of pressurized fluid is supplied to the pressure control valve 28 for moderately increase the fluid pressure. Accordingly, the fluid pressure in the working chamber 26d of respective cylinders 26 is also moderately increased.

When the line pressure is increased across the neutral pressure P_(N), the operational one-way check valve 172 opens to permit the fluid flow through the drain line 37. Therefore, the pressure control valve 28 is enabled to control the fluid pressure in the working chamber.

On the other hand, as set forth above, the microprocessor 110 maintains the suspension control command values at the initial values P_(FL) ', P_(FR) ', P_(RL) ' and P_(RR) ' until the initial period timer value N_(I) reaches the set initial timer value N_(SI). Therefore, the suspension control signal values I_(FL), I_(FR), I_(RL) and I_(RR) output from the driver circuits 128FL, 128FR, 128RL and 128RR are maintained at the values corresponding the initial suspension control command values P_(FL) ', P_(FR), ', P_(RL) ' and P_(RR) '. As will be appreciated from the foregoing discussion, since the initial suspension control command values P_(FL) ', P_(FR) ', P_(RL) ' and P_(RR) ' are set at higher than the neutral pressure P_(N) for the off-set pressure values PO_(j).

After expiration of the ON-set period as defined by the set initial period value N_(SI), the flow control valve 171 is operated into open position by switching the flow control signal CS₂ into LOW level. Therefore, increased or non-limited flow rate of pressurized fluid is supplied to the pressure control valves 28. At this condition, the off-set command values PO_(j) is gradually reduced to zero through the steps 1022 and 1024. Then, height regulating control, anti-rolling control and anti-pitching control is performed on the basis of the height indicative data H_(j), the lateral acceleration indicative data Y_(G) and the longitudinal acceleration indicative data X_(G). Manner of height regulation control, anti-rolling control and anti-pitching control have been disclosed in various foregoing co-pending U.S. Patent Applications. Therefore, all disclosures touching height regulation control, anti-rolling control, anti-pitching control and so forth discussed in the associated U.S. Patent Applications are herein incorporated by reference for the sake of disclosure.

In the height regulation control, since the initial stage indicative flag F₁ is set at the step 1018, the target height indicative value H_(S) is set at a value H₀ which is smaller than the normal value H₁. Furthermore, the interval of height adjustment as defined by the height adjustment threshold value T_(H) is set shorter period T₀. On the other hand, the height variation value ΔH is set at greater value ΔH₀ than the normal value ΔH₁. By this initial setting, the quick regulation of the vehicular height can be achieved.

On the other hand, when the ignition switch 140 is turned OFF, the engine stops running. Accordingly, the fluid pump 161 in the pressure source unit 16 cease driving. However, by the effect of the one-way check valve 174 in the pressure holding mechanism 17, the abrupt drop of the line pressure at the pressure control valves 28 can be successfully prevented.

On the other hand, since the self-holding signal S_(S) is maintained HIGH level for the given OFF-set period. Therefore, power supply for the control unit 100 is maintained for maintaining control operation. At the beginning of the OFF-set period, i.e. upon turning OFF the ignition switch 140, the short period re-starting indicative flag F_(S) is set at the step 1074. Subsequently, during the OFF-set period, the fluid pressure in the working chamber is gradually adjusted to the set pressure P_(K). If the ignition switch 140 is again turned ON before expiration of the given OFF set period expires, then the answer at the step 1002 becomes negative to initiate derivation of the off-set pressure PO_(j) at the step 1020. Namely, assuming that the off-set pressure immediately before turning ON of the ignition switch 140 is PO_(j) ", the off-set pressure initially derived can be expressed by:

    PO.sub.j =PO.sub.j "-P.sub.N

As can be appreciated from the foregoing equation, at this time, since the vehicle is not running, the anti-rolling and anti-pitching command values P_(YG) and P_(XG) are maintained zero. Therefore, the suspension control command values are maintained at P_(j). Thereafter, the initial stage indicative flag F_(I) is set at the step 1018. As can be appreciated, at this time, since the process steps 1004 through 1016 are skipped, the flow control valve 171 is held open. Therefore, quick increase of the fluid pressure at the pressure control valve can be achieved.

In the preferred embodiment, upon re-starting, the short period re-starting indicative flag F_(S) as set and the suspension control commands P_(j) may be stored in a back-up memory. This may prevent the status of the short period re-starting indicative flag F_(S) from being changed even when the power source voltage is fluctuated due to high power consumption during engine cranking operation. This may assure re-starting of suspension control at the corresponding state of control.

On the other hand, when the ignition switch 140 is maintained at OFF position for a period longer than the OFF-set period defined by the second end timer threshold value T_(ES2), both of the fail-safe signal CS₁ and the self-holding signal S_(S) are set at LOW level upon expiration of the OFF-set period. Subsequently, when the line pressure drops to the neutral pressure P_(N), the operational one-way check valve switches the valve position to flow blocking state. At this position, since part of the fluid in the drain line 37 flows into the back pressure absorbing pressure accumulators 24F and 24R, the pressure at the pressure control valve is maintained slightly lower than the neutral pressure P_(N).

On the other hand, when the failure of the system is detected by the fail detector 130, the fail-safe signal CS₁ is switched into LOW level to place the fail-safe valve 18 at the flow blocking state. At the same time, since the self-holding signal S_(S) is switched into LOW level, the power supply for the control unit 100 is shut down. At the same time, the pressure source unit 16 is shut down to stop pressure supply. Then, since the inlet port of the pressure control valve comes into communication with the operational check valve 172 via the flow restriction orifices 183. Therefore, the pilot pressure P_(P) corresponding to the line pressure in the supply line drops to the neutral pressure P_(N) to cause shutting down of the operational one-way check valve. Therefore, the hydraulic circuit across the working chamber and the pressure control valves becomes closed. Therefore, the pressure in the closed circuit can be maintained slightly below or in the vicinity of the neutral pressure.

At this condition, when the vehicular load is substantially standard, the vehicular height can be maintained in the vicinity of the target height. Furthermore, shock absorbing effect substantially equivalent to the conventional passive type shock absorber can be achieved by inducing fluid flow with flow restriction in the closed circuit.

While the present invention has been discussed in terms of the preferred embodiment of the invention, the invention can be embodied in various fashion. Therefore, the invention should not be appreciated to be specified to the specific embodiment discussed hereabove. Therefrom the invention should cover all possible embodiments and modifications which can implemented without departing from the principle of the invention.

For instance, though the pressure holding mechanism in te shown embodiment comprises the electromagnetically operable flow control valve and the flow restrictive orifice, the flow control valve may also comprise a pilot operated valve. Furthermore, the position of the pressure holding mechanism may not be specified to the shown position but can be any appropriate position convenient for equivalent flow control operation. Furthermore, the flow restriction orifice is not essential to be provided. For example, when a flow restriction orifice is provided between the P port and A port of the fail-safe valve, the flow restriction orifice parallel to the flow control valve can be neglected. In addition, though the shown embodiment employs the engine driven fluid pump, it can be replaced with an electrically driven fluid pump. Also, the present invention may be implemented any type of pressure control valve or flow control serve unit performing equivalent pressure control function. 

What is claimed is:
 1. An active suspension system comprising:a suspension system disposed between a vehicular body and a road wheel for damping energy for causing relative displacement between the vehicular body and the road wheel, said suspension system including means for defining a variable pressure chamber; a fluid circuit for circulating pressurized fluid across said variable pressure chamber, said fluid circuit including a pressurized fluid source and a pressure control valve means for adjusting fluid pressure within said variable pressure chamber across a predetermined neutral pressure; a flow control valve network disposed between said pressurized fluid source and said pressure control valve means in said fluid circuit for establishing and blocking fluid communication between said pressurized fluid source and said pressure control valve means; a sensor means for monitoring vehicular driving parameter affecting vehicular attitude for producing a sensor signal indicative thereof; a power supply circuit for supplying an electric power to said fluid pressure source for driving the latter; a control means responsive to said sensor signal for controlling said pressure control valve for adjusting pressure in said variable pressure chamber for regulating vehicular height and vehicular attitude, said control means powered by the electric power supplied by said power supply circuit, said control means designed to maintain operation for a predetermined first period after shutting down of power supply from said power supply circuit for adjusting the fluid pressure in the vicinity of said neutral pressure and operating said flow control valve network at said blocking position for maintaining the pressure in said variable pressure chamber at the pressure in the vicinity of the neutral pressure, said control means being responsive to initiation of power supply for maintaining said flow control valve network at flow blocking position for a predetermined second period of time so as to increase line pressure to be supplied to said pressure control valve at a limited rate, and said control means being responsive to resumption of power supply within said first period for maintaining said flow control valve network at fluid communication establishing state and setting instantaneous fluid pressure as an initial pressure for starting suspension control.
 2. An active suspension system as set forth in claim 1, wherein said control means normally derives suspension control signal on the basis of said sensor signal so as to regulate vehicular height and vehicular attitude, said control means setting said control signal for a predetermined fixed value during said second period.
 3. An active suspension system as set forth in claim 2, wherein said sensor means comprises a height sensor for monitoring vehicular height level to produce a height level indicative sensor signal, and said control means derives said suspension control signal on the basis of a difference between said height level indicative signal value and a target height representative value, said control means set said target height representative value at a first normal value at a normal operation and a second smaller value smaller than said first normal value during said second period.
 4. An active suspension system as set forth in claim 3, wherein said control unit further sets an interval of height regulation control cycle at a first normal period and a second reduced period during said second period, which second reduced period is set to be shorter than said first normal period.
 5. An active suspension system as set forth in claim 3, wherein said control unit further sets a height adjustment magnitude for each height adjustment cycle, which height adjustment magnitude is set at a first normal value in the normal state of height control and a second increased value during said second period.
 6. An active suspension system as set forth in claim 1, wherein said control unit adjusts said fluid pressure in said working chamber within a predetermined range set about a set pressure, during said first period.
 7. An active suspension system as set forth in claim 1, which further comprises a fail-safe valve in said fluid circuit, and a fail detecting means for monitoring operation to detect failure of the active suspension system for producing a fail indicative signal, said control unit normally operates said fail safe valve to a first open position and responsive to said fail indicative signal for blocking fluid communication therethrough for forming closed circuit across said pressure control valve and said working chamber.
 8. An active suspension system comprising:a suspension system disposed between a vehicular body and a road wheel for damping energy for causing relative displacement between the vehicular body and the road wheel, said suspension system including means for defining a variable pressure chamber; a fluid circuit, including a supply line for supplying pressurized fluid and a drain line for driving pressurized fluid, for circulating pressurized fluid through said supply line and said drain line across said variable pressure chamber, said fluid circuit including a pressurized fluid source and a pressure control valve means for adjusting fluid pressure within said variable pressure chamber across a predetermined neutral pressure; a flow control means disposed between said pressurized fluid source and said pressure control valve means in said fluid circuit, said flow control means being operable between a first flow control mode for supplying line pressure for said pressure control valve at a first maximum flow rate and a second flow control mode for supplying line pressure for said pressure control valve at a second limited flow rate which is smaller than said first maximum flow rate; a sensor means for monitoring vehicular driving parameter affecting vehicular attitude for producing a sensor signal indicative thereof; a power supply circuit for supplying an electric power to said fluid pressure source for driving the latter so as to enable supply of pressurized fluid to said fluid circuit; a control means responsive to said sensor signal for controlling said pressure control valve, said control means being operable in a first normal mode for adjusting pressure in said variable pressure chamber for regulating vehicular height and vehicular attitude, a second turn OFF transition mode initiated in response to shutting down of said power supply circuit for operating said flow restriction means at said first flow control mode for adjusting fluid pressure in said working chamber toward a first set pressure set in the vicinity of said neutral pressure during a predetermined first turn OFF transition period, and a third turn ON transition mode for operating said flow restriction means at said second flow control mode for adjusting fluid pressure in said working chamber toward a second set pressure which is set higher pressure than said neutral pressure in a given magnitude during a second ON transition period, and said control means being responsive to resumption of power supply within said first turning OFF transition period for maintaining said flow control valve means at said first flow control mode and setting instantaneous fluid pressure as an initial pressure for starting suspension control.
 9. An active suspension system as set forth in claim 8, wherein said control means normally derives suspension control signal on the basis of said sensor signal so as to regulate vehicular height and vehicular attitude, said control means setting said control signal for a predetermined fixed value during said second turning ON transition period.
 10. An active suspension system as set forth in claim 9, wherein said sensor means comprises a height sensor for monitoring vehicular height level to produce a height level indicative sensor signal, and said control means derives said suspension control signal on the basis of a difference between said height level indicative signal value and a target height representative value, said control means set said target height representative value at a first normal value at a normal operation and a second smaller value smaller than said first normal value during said second turning ON transition period.
 11. An active suspension system as set forth in claim 10, wherein said control unit further sets an interval of height regulation control cycle at a first normal period and a second reduced period during said second turning ON transition period, which second reduced period is set to be shorter than said first normal period.
 12. An active suspension system as set forth in claim 10, wherein said control unit further sets a height adjustment magnitude for each height adjustment cycle, which height adjustment magnitude is set at a first normal value in the normal state of height control and a second increased value during said second turning ON transition period.
 13. An active suspension system as set forth in claim 8, wherein said control unit adjusts said fluid pressure in said working chamber within a predetermined range set about a set pressure, during said first turning OFF transition period.
 14. An active suspension system as set forth in claim 8, which further comprises a fail-safe valve in said fluid circuit, and a fail detecting means for monitoring operation to detect failure of the active suspension system for producing a fail indicative signal, said control unit normally operates said fail safe valve to a first open position and responsive to said fail indicative signal for blocking fluid communication therethrough for forming closed circuit across said pressure control valve and said working chamber. 